Cartridge fuse



Nov. 2l, 1939.v P. vA HUNTER ET AL CARTRIDGE FUSE Filed Jan. 24, 1958 Fig. I.

Patented Nov. 21, 1939 UNITED STATES PATENT OFFICE CARTRIDGE FUSE Application January 24, 1938, Serial No. 186,728 In Great Britain January 30, 1937 2 Claims.

This invention relates to cartridge fuses of the kind comprising a number of fusible elements enclosed within the cartridge, embedded in powdered or granular material and connected in parallel, each element having some means, such as the formation of a waist, for fixing the region in which the element starts to fuse.

In such cartridge fuses it has been found that the difliculty of making a satisfactory fuse increases as the rated carrying current of the fuse is raised.` This has led, in fuses of the larger sizes, to sub-division by placing the fusible elements either singly or in groups in a number of separate chambers in the same cartridge.

We have found that, by carrying out the subdivision to a substantially greater degree than has been practised hitherto, Very advantageous results can be obtained. By sub-division we refer here to the number of separate chambers into which the cartridge is divided, rather than to the number of fusible elements. In many cases there will be one element in each chamber, but this is not necessarily the case, and it is the subdivision into a relatively large number of chambers which gives the advantageous result. It has been found experimentally that the effect of grouping of a number of small fusible elements in parallel in the same chamber in a fuse of the kind referred to is to produce an increase in the severity of the explosion on short circuit as compared with the case of a single element of the same total cross section and of the same fusible material.

In accordance with the invention, the subdivision is carried out so that one chamber corresponds to about amperes of rated carrying current as a maximum value. This gure of 15 is to be regarded as a general indication rather than a rigid standard. At the lower end of the range of rated carrying current values the figure of 15 may be exceeded somewhat. At the higher end of the range it may, with advantage, be taken at a lower value. For instance, at 60 amperes of rated carrying current a fuse might have three chambers, (i. e. amperes per chamber) While at 500 amperes of rated carrying current a fuse might be divided into chambers, (i. e. 12.5 amperes per chamber). As an approximation this range of values may be represented by the empiric formula where N is the number of chambers and C is the rated carrying current in amperes. This formula (Cl. 20G-131) gives a value for N which is a lower limit; this limit may be exceeded in many cases.

The rated carrying current, is dependent upon the material and dimensions of the fusible element. Since there is a definite ratio, known as 5 the fusible factor, between the minimum fusing current and the rated carrying current and the minimum fusing current is determined by the physical properties and dimensions of the fusible element for the types of fuse with which the invention is concerned. Appropriate fusing factors are, for instance, given in British Standards Specification 88-1937, where, for different categories of duty for short circuit protection, the factor ranges from 2.0 to 1.6 and, for fuses in- 15 tended for both over current and short circuit protection, 1.4 is given as the maximum value.

Investigations have been made involving repeated short circuit tests at high rupturing capacity. These have been carried out with circuit resistance varied so as to provide a large range of prospective (as defined in British Standard Specilication 88-1937) current. It has been shown by these tests that for each fuse, the actual current at the breaking of the circuit increases when the prospective current of the circuit is increased, but this increase of actual current proceeds at a decreasing rate until finally a maximum current is reached, which is not exceeded however large the prospective short circuit current may be made for the given circuit voltage and inductance. This maximum value, which may be called the saturation current, has been found to be greatly reduced for a given rated carrying current by using fuses with the high degree of sub-division hereinbefore described.

By taking this high degree of sub-division it is found possible to use, with satisfactory results, a form of enclosure for the separate chambers which is of comparatively light structure, while possessing ample strength, and this is satisfactory, when used in the appropriate number, for fuses of high rated carrying currents.

'Ihe fuse cartridge may be built upy of an assembly of separate tubes held together, or by forming a number of passages through a block of material or a combination of these. An example of the last mentioned kind of construction is shown in the accompanying drawings which illustrate by way of example a cartridge fuse with a rated carrying current of 150` amperes, sub-divided in accordance with the present invention into fifteen chambers.

Figure 1 is a section, on a central longitudinal 55 plane, of the complete cartridge and Figure 2 is an end View of the cartridge. The approximate dimensions of the cartridge and its parts will be recognised vif it be assumed that the diameter of the barrel is 50 millimetres.

Each of the fusible elements is a silver wire of .028 inch diameter with a waist in the centre of the length one-quarter of an inch and of .01 inch diameter. Each element l is enclosed in a glass tube 2 with a lling of sand 3. Each tube is plugged at the end with plaster of Paris as shown at il. Each tube is enclosed in a cylindrical chamber in the barrel 5. The space between the tube 2 and the wall of the chamber is packed with sand 6 and each end of the chamber is sealed with plaster of Paris as at l. The ends of the wires i are soldered to the end caps 8.

Owing to the very small bore compared with the length of each chamber which the extensive sub-division leads to it is readily practicable for each chamber tc be made and plugged at the ends in such a manner that the walls and plugs will resist the explosive forces. Accordingly, these parts of the cartridge are self-supporting and in particular do not rely on the strength oi the attachment of the end caps of the cartridge to the body and the manufacture of the cartridge body and the caps may be made an entren ely simple process.

It can be calculated that, for given circuit conn ditions, an increase in the number of similar fusible elements connected in parallel in a fuse, of the kind hereinbefore referred to, is accompanied by a decrease in the current per element at the coinmencement oi rupture on short circuit. Experiments show that this advantageous result is not accompanied by a reduction in the explosive forces unless sub-division into separate chambers is adopted. It is probable, however, that it is a contributory cause oi the advantages of subdivision when carried to the degree set forth in accordance with the present invention.

It is believed that an important factor in the obtaining of these advantages is the greater facility for arc quenching by cooling, by the surrounding powdered or granular medium, when using very small fusible elements in separate chamber. This results in a reduction of the total energy liberated in a group of chambers at the blowing of the fuse and hence a lower pressure in each chamber in a fuse of a given rated carrying current. This reduction of pressure, used in combination with the fact that the strength of a tube to resist internal pressure is a function of the ratio of the internal diameter to the thickness of the wall, permits of the utilisation of thin walls in a small chamber without danger of bursting at the blowing of the fuse. Other factors in addition to these may assist in the obtaining of the advantageous results.

What we claim as our invention is:

l. A cartridge fuse comprising a number of chambers and fusible elements surrounded by finely divided material in said chambers, each element having means for xng the region in which fusion starts, the number of chambers being not less than a value N obtained from C being the rated carrying current of the oartridge.

2. A cartridge fuse comprising a number of chambers and fusible elements surrounded by iinely divided material in said chambers, each element having a portion of reduced cross section near the centre of the length of the chamber, the number of chambers being not less than a value N obtained from C being the rated carrying current or" the cartridge.

PHILIP VASSAR HUNTER. ALFRED WHITLEY METCALF. 

